In order that a lid swingably mounted on a stationary box, container, rack or the like smoothly open and securely close at a given position a mechanism is required. Conventional devices or mechanisms have hitherto used a magnetic device for holding a lid at a given closed position and an air damper has been used for performing smooth opening of a lid. However, since the function of such a magnetic device differs from that of the damper, both of these had to be employed for concurrently performing their functions. Furthermore, such a magnetic device for holding or locking a lid at a given closed position has a drawback in that the lid has to be pulled manually with a force greater than the attracting force of the magnetic device. The air damper for performing smooth opening also has a drawback in that it requires a large space.